Parkinson’s disease comes with two main buckets of possible symptoms. One affects your ability to move and leads to motor issues like tremors and rigid muscles. The other bucket has non-motor symptoms, like pain, loss of smell, and dementia.
You may not get all the symptoms -- and you can’t predict how bad they’ll be, or how fast they’ll get worse. One person may have slight tremors but severe dementia. Another might have major tremors but no issues with thinking or memory -- and someone else may have severe symptoms all around.
On top of that, the drugs that treat Parkinson’s work better for some people than others. All that adds up to a disease that’s very hard to predict.